The present invention relates generally to ceramic foams and their methods of preparation and particularly to such ceramic foams as find use in the filtration of molten metals such as molten aluminum.
The preparation of porous ceramic articles such as ceramic foams from the impregnation of open-celled organic sponges or foam materials has been disclosed in the prior art, as represented by U.S. Pat. No. 3,090,094 to Schwartzwalder et al. and U.S. Pat. No. 3,097,930 to Holland. Likewise, the use of such articles as filters for molten metal and particularly for the filtration of molten aluminum and copper has been disclosed in U.S. application Ser. No. 430,229 for "Improved Molten Metal Filter" by Michael J. Pryor et al., filed Jan. 2, 1974.
Though, as noted above, the preparation of porous ceramic articles is known, the successful use of such articles in exacting functions such as the filtration of molten metal requires that the article possess particular physical and chemical properties. Specifically, the ceramic foam articles requires a certain permeability and structural uniformity to efficiently filter molten metal at commercially acceptable rates and purity levels. As a corollary property, the foam material must withstand chemical attack from the molten metal to facilitate its extended use as a filter.
As noted above, the prior art suggests the preparation of ceramic foam materials. Particularly, U.S. Pat. No. 3,111,396 to Ball suggests that an organic polymer foam impregnated with a refractory material may then be compressed by passage through preset rollers to effect the removal of excess refractory. This technique, which is comparable to a wide variety of conventional expulsion techniques used in the art suffers from an inherent disadvantage in that the slurry is not completely uniformly distributed through the body of the article. Thus, the outer area of the article tends to be more thinly coated with slurry than that near the center line. Such defects are particularly extended at the extremes of the permeability range found suitable for use in the preparation of molten metal filters; thus, bodies possessing a high permeability tend to have undesirably weak surfaces and edges whereas, bodies possessing relatively low permeability tend to exhibit undesirable center line blockage. Both of the aforenoted defects render the resulting foams unsuitable for use in the filtration of molten metal.